Builds a complete, usable map from the string passed in seed. Each triplet
composed of seed/map_w/map_h should build the same map on any computer,
in any context, for a given version of the game (no backwards compatibility).

Return value: new map on success, NULL on failure.

Function: char *lw6gen_seed_new()

Generate a new random seed. The seed is composed of letters and numbers.

Return value: newly allocated string.

Function: char *lw6gen_seed_normalize(const char * seed)

seed: the seed to normalize

Builds a normalized seed from an arbitrary string. The idea is to
avoid sending strange stuff on the network and/or storing strange
stuff in config files, so we process correct strings only